396 



BRAIN MECHANISMS AND LEARNING 



rostral mesencephalon was tested by transecting the brain stem at the mid- 

 collicLilar level. In these animals, a rapid dechne of post-rotational ocular 

 movements was also obtained (Fig. 2). Such results indicated that habitua- 

 tion of post-rotatory nystagmus does not require the thalamic station of 

 the vestibular pathway either, and pointed to the lower brain stem as the 



Fig. 2 



Elcctroinycigrdphic recordings of post-rotatory nystagmus. (A) Nystag- 

 mus of the first experimental rotation. (B) After habituation had been 

 established with thirty rotations an extensive electrolytic lesion which 

 rendered the cat unconscious was made in the mesencephalic tegmen- 

 tum. (C) The first rotation after the lesion elicited nystagmus which 

 diminished at a faster rate than in the aw^ike intact animal with successive 

 trials. D represents the recording obtained after fifteen trials with the brain 

 stem lesion. 



probable source oi inhibitory mechanisms which prevent the discharge 

 of extra-ocular motoncurones after repeated vestibular stimulation. 

 Subsequent experiments demonstrated that lesions in the mesencephalic 

 reticular formation did not prevent habituation of post-rotatory nystag- 

 mus but that lesions placed in the pontine tcgmcntiun prevented the 

 habituatory process. 



